Alturas County was enormous. When created in 1864, it included practically all the land between the Snake and Salmon rivers in the south-central part of the state.
Now, when you create a new county, you always like to name a county seat. About the only thing you really need for a county seat is a town. But Alturas County, with all those thousands of acres, didn't have a real town, so the legislature invented one. They called it Esmeralda, and it was located on a beautiful plateau near the South Fork of the Boise River, about a mile below what is now Featherville.
Esmeralda was never more than a handful of slap-dash cabins occupied by some early-day prospectors. Its moment of fame was little more than a moment. Two months after it was named the county seat of Alturas County, the county commissioners moved their operation to the new town of Rocky Bar, where gold had just been discovered. The commissioners and prospectors left Esmeralda, and the town just disappeared.
So did the county, eventually. Alturas County existed for over thirty years, but increased population within its boundaries prompted the legislature to split it up into smaller counties in 1896. Smaller, but not small counties. Blaine, Camas, Gooding, Lincoln, Jerome, Elmore, and Minidoka counties were all carved from Alturas.
The map, courtesy of the Idaho Genealogical Society, shows the original boundaries of Alturus County and the counties that split off from it.